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An examination of the flame spread limits in a dual fuel engine
PERGAMON
Applied Thermal Engineering 19 (1999) 1071±1080
An examination of the ¯ame spread limits in a dual fuel engine O. Badr 1, G.A. Karim *, B. Liu The University of Calgary, Department of Mechanical Engineering, Calgary, Alta., Canada, T2N 1N4 Received 3 March 1998; accepted 12 October 1998
Abstract The performance of a gas-fuelled diesel engine (dual fuel) is examined at light load and an eective threshold limit to the combustion of the gaseous fuel through bulk ¯ame spread is identi®ed. The relationship of such a limit to some of the key operating parameters is then discussed. A comparison between the measured values of the limit with those corresponding to the lower ¯ammability limits of the gaseous fuel when evaluated under the prevailing cylinder conditions during pilot diesel fuel ignition showed similar trends. It is suggested that such a similarity may form a basis for estimating the lean operational limits for dual fuel combustion in engines. A simple approach for estimating the limiting equivalence ratio for the apparent bulk ¯ame spread limit is described for a methane-fuelled dual fuel engine. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The use of alternative gaseous fuels in engines for the production of power has been increasing worldwide. This has been prompted by the cleaner nature of their combustion compared with conventional liquid fuels as well as their relative increased availability at attractive prices. Their use in engines is expected to increase with the ever tightening of exhaust emission regulations. Diesel engines, with appropriate relatively simple conversion, can be made to operate on gaseous fuels eciently. Such engines, which are called `dual fuel engines', usually have the gaseous fuel mixed with the air in the engine cylinders, either through direct mixing in the intake manifold with air or through injection directly into the cylinder. The resulting mixture * Corresponding author. Tel.: +1-403-220-5775; fax: +1-403-282-8406; e-mail: [email protected] 1 Currently at Qatar University, Doha, Qatar. 1359-4311/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 8 ) 0 0 1 0 8 - 2
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after compression is then ignited through the injection of a small amount of diesel fuel (the pilot) in the usual way. This pilot liquid fuel can autoignite readily to provide ignition sources for subsequent ¯ame propagation within the surrounding gaseous fuel mixture. Most current dual fuel engines are made to operate interchangeably, either on gaseous fuels with diesel pilot ignition or wholly on liquid fuel injection as a diesel engine. Accordingly, a dual fuel engine tends to retain most of the positive features of diesel operation. It may even surpass occasionally those of the diesels, producing higher power outputs and eciencies. This is achieved without signi®cant smoke or particulates emission and with reduced NOx production [1], while having reduced peak cylinder pressures and quieter operation. On the other hand, dual fuel operation may produce knocking, particularly with very high power outputs or raised intake temperatures, even with the knock-resistant fuel, methane. Fortunately, the knocking region with natural gas, which is mostly made up of methane, is out of most common operations, unless highly turbocharged engines are used, or the natural gas composition includes signi®cant concentrations of higher hydrocarbon fuel components. A notable feature of dual fuel engine operation is its inferior performance at very light load, especially when using small pilots, manifested by increased speci®c energy consumption, cyclic variations and higher emissions of CO and HC relative to the corresponding diesel operation. These limitations arise primarily from the fact that ¯ames originating from ignition regions within the pilot envelope cannot propagate fast and far enough within the time available to consume the entire fuel-lean mixture [2±4]. Such performance can be improved through measures such as the use of high enough gaseous fuel concentrations to permit ¯ame propagation, larger pilots, heating the intake charge, partial throttling the air component, variable pilot injection timing, optimal strati®cation of gaseous fuel admission and lower engine speeds [5]. Successful implementation of such measures requires a knowledge of the limiting concentration of the gaseous fuel in air beyond which favourable operation begins to be encountered. The present contribution examines the performance of a dual fuel engine at light load and identi®es an eective threshold limit to the consumption of the gaseous fuel with bulk ¯ame spread. The relationship of such a limit to some of the key operating parameters is then discussed. A comparison is also made between the measured values and those corresponding to the lower ¯ammability limits of the fuel evaluated at the cylinder temperature and pressure at the time of pilot ignition. It is shown that the similar trends between these two sets of limits suggest that they may form a basis for estimating the lean limits for dual fuel combustion and satisfactory engine operation.
2. Apparatus The experimental results in this investigation were obtained from two single cylinder, four stroke, watercooled, direct injection, normally aspirated laboratory dual fuel engines having 105 mm bore and 152.5 mm stroke with either a compression ratio of 14.2 and pilot injection timing of 208 for engine A or 14.7 and 188, respectively, for engine B. The intake mixture temperature was varied either through cooling for engine A from 293 to 250 K or through
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heating for engine B from 288 to 400 K. The fuel employed was technically pure methane and the diesel fuel had a cetane number of around 45. The engines were run at 1000 rpm. The total equivalence ratio during dual fuel operation which accounted for the contributions of the methane and diesel pilot fuels was calculated as the measured mass air ¯ow rate relative to the corresponding mass rate required for the stoichiometric combustion of the two fuel components. 3. Lean combustion at light load The autoignition of the injected pilot liquid diesel fuel provides ignition centres for turbulent ¯ame propagation throughout the lean homogeneous gaseous fuel±air mixture. Engine performance at light load tends to improve with increased admission of the gaseous fuel. The extent of the improvement appears to be dependent on the total equivalence ratio (i.e. based on the pilot and gaseous fuels). Fig. 1 shows the variations with total equivalence ratio of the concentrations of unconsumed methane and carbon monoxide in the exhaust gas for the engine for dierent pilot quantities. It can be seen that there is a limiting equivalence ratio beyond which the exhaust emissions of the carbon monoxide and the unconverted methane become virtually unaected by the pilot quantity. This is indicative of the equivalence ratio limit for successful ¯ame propagation from the pilot ignition centres. Fig. 2 shows schematically the typical variations in the extent of exhaust emissions of carbon monoxide and methane with the overall equivalence ratio for a ®xed pilot quantity. Broad operational regions may be identi®ed. The ®rst region is associated with extremely low gaseous fuel admission where the exhaust emissions of carbon monoxide and the fraction of the methane consumed are very small. In the second region, following an increased admission of the gaseous fuel, the consumption of the methane and the production of carbon monoxide begin to increase rapidly with the continued increased admission of the methane. Later on,
Fig. 1. The variations of the exhaust gas concentrations of methane and carbon monoxide with total equivalence ratio for dierent pilot fuel quantities at ambient intake conditions and 1000 rpm.
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Fig. 2. Schematic variations of the uncoverted methane and carbon monoxide concentrations in the exhaust with total equivalence ratios showing the dierent operational regions.
these begin to decrease in regions III and IV. These limiting values of equivalence ratio can be identi®ed as f1, f2 and f3 which may signify the start of signi®cant local partial oxidation, ¯ame initiation and the spread of propagating ¯ames within the gaseous fuel±air charge, respectively. The complex chemical and physical interactions that take place to produce these regions require the consideration of a number of related processes. These would include the preignition reaction activity of the gaseous fuel±air mixture during compression, the pilot injection processes and subsequent formation of the ¯ammable envelope, progressive reactions during the ignition delay of the pilot, formation of ignition centres and subsequent reactions with the gas±air mixture that may lead to partial or complete ¯ame propagation [6]. When operating with very fuel lean mixtures at light load most of the energy release comes from the combustion of the pilot and the gaseous fuel entrained within its envelope as well as adjacent reacting zones where high temperature may evolve. The contribution of the bulk surrounding lean gaseous fuel±air mixture to the energy release remains small [5]. For less lean mixtures, the concentration of the gaseous fuel may become suciently high to permit ¯ame propagation throughout the entire charge within the time available to contribute signi®cantly at a more gradual rate to the overall energy release [3]. Generally, the oxidation of a fuel such as methane proceeds sequentially via the formation of formaldehyde followed by carbon monoxide and the subsequent conversion to carbon dioxide and water vapour. For suciently fuel rich mixtures yielding high temperatures, good conversion of the methane±air mixture to completion takes place with little carbon monoxide and unconverted methane appearing in the exhaust. For less rich mixtures producing on combustion moderately high temperatures, a substantial amount of the carbon monoxide produced cannot be converted in the time available to carbon dioxide. However, for suciently lean mixtures, the charge temperature may be so low that no signi®cant reactions proceed, leaving the bulk of the methane unconverted and producing insigni®cant amounts of carbon monoxide in the exhaust. In region I of Fig. 2, associated with very low equivalence ratios, carbon monoxide is produced at very low levels and comes mainly from the combustion of the pilot. The contribution made by the surrounding zone of the gaseous fuel±air mixture is very small. In
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region II, some of the exhaust carbon monoxide originates increasingly from the preignition reactions of the gaseous fuel within the unburned zone, yet they are incapable of leading to ¯ame propagation within the time available, in spite of the presence of ignition centres. Further increases in equivalence ratio beyond the value f2 permit some ¯ame propagation within the methane±air mixture. In region III, further increases in the admission of the gaseous fuel produces extentions to the size of both the pilot envelope and the adjacent reacting zone and extends the ¯ame propagation into a larger fraction of the cylinder charge. With increased gaseous fuel admission the ¯ame propagation continues to extend into further regions of the charge until at f3 it extends essentially to all parts of the combustion chamber. Further increases in the gaseous fuel concentration within region IV produce essentially proportionally high rates of heat release, leading to high cylinder pressures and increased power output [6].
4. Combustion limits The existence of combustion limits in gas fuelled diesel engines has been recognized by dierent workers over the years. Elliot and Davis [2] reported such apparent limits in a dual fuel engine when using a number of dierent gaseous fuels. Karim and Wierzba [7] showed that the operational limits in a gas-fuelled spark ignition engine were amenable to correlation in terms of the charge mean temperature at the time of passing the spark. Badr et al. [8] de®ned for a gas-fuelled spark ignition engine two operational mixture limits and related them to the corresponding values under quiescent conditions calculated for the prevailing conditions within the cylinder at the time of passing the spark. A knowledge of the lean limits of combustion in gas-fuelled engines would obviously help in improving the design and operation of engines, particularly when embarking on the conversion of conventional diesel engines to gas fuelled operation. There is a need to establish whether simple guidelines for estimating such limits can be derived and to estimate their values in terms of operating conditions for a speci®c engine. The volumetric concentration of the gaseous fuel in air at the ¯ame spread limit (FSL) which corresponds to the equivalence ratio f3 of Fig. 2, as indicated earlier, identi®es the boundary for the commencement of satisfactory engine operation and improved emissions. This limit represents the minimum concentration of the gaseous fuel in air for which ¯ame propagation appears to spread throughout the entire cylinder charge. Such a limit for a speci®c engine is obtained experimentally when the emissions of the unconverted gaseous fuel becomes essentially independent of the pilot quantity employed. On the other hand, the equivalence ratio of the charge associated with the observed peak value of the concentrations of carbon monoxide exhaust emissions, corresponding to f2 in Fig. 2, may be considered as indicative of the commencement of some limited ¯ame propagation into the adjacing mixture. These limits need to be calculated for the gaseous fuel on the basis of the air remaining after accounting for its reduction following the complete oxidation of the pilot fuel. This lean limit for ¯ame spread in the dual fuel engine, despite obvious dierences, is reminiscent of the limit for ¯ame propagation within corresponding quiescent mixtures commonly known as the lean ¯ammability limit (LFL). Accordingly, it can be suggested that
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Fig. 3. Variations of the experimentally established ¯ame spread limit (FSL) with the pilot quantity employed for methane operation at a compression ratio of 14.2:1 and 1000 rpm. The corresponding ¯ame initiation limit values are also shown.
the observed limit (FSL) in the engine should be relatable to the corresponding lower ¯ammability limit (LFL) of the gaseous fuel under temperature and pressure values similar to those of the gaseous fuel±air cylinder charge during pilot fuel ignition, albeit under quiescent conditions. Fig. 3 shows the observed variations of the ¯ame spread limits (FSL) with changes in the pilot quantity derived from experimental data for the test engine with an estimated uncertainty of 5%. The lowering of the limit with the increase in the pilot quantity is expected due to a number of contributing factors. These include a greater energy release on ignition, correspondingly improved pilot injection characteristics, a larger size of pilot mixture envelope with a greater entrainment of the gaseous fuel, a larger number of ignition centres requiring shorter ¯ame travels, higher rates of heat transfer to the unburned gaseous fuel±air mixture and an increased contribution of hot residual gases. The ¯ame initiation limit shown in Fig. 3, exhibits a similar trend. Fig. 4 shows similarly how the operational limit (FSL) of the two dual fuel engines employed for a ®xed pilot quantity varies with changes in the intake charge temperature. As expected, the limit is lowered with the increase in intake temperature. This
Fig. 4. Variations of the experimentally established ¯ame spread limit (FSL) with the intake temperature for the two engines employed.
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trend arises mostly from the corresponding increase in the mean charge temperature during pilot ignition. 5. Correlations of the observed limits It has been shown that homogeneous quiescent mixtures of most common gaseous fuels in air at their corresponding lean ¯ammability limit concentrations produce on combustion approximately the same ¯ame temperature for a wide range of temperatures and pressures [9]. The observed ¯ame spread limits in an engine may be tested to establish whether they can be correlatable on a similar basis. The gaseous fuel±air engine mixture is at a condition that corresponds to the average cylinder temperature and pressure at the time of ¯ame initiation (i.e. at the point of pilot ignition). These conditions which are a function of most engine operating variables such as intake temperature and pressure, compression ratio, pilot injection quantity and timing, heat transfer and the physical properties of the mixture were evaluated according to the following three dierent possible simplifying interpretations of the processes: i. The temperature is considered to be that following compression calculated at TDC conditions with the assumption that pilot ignition contributes negligibly little to the value of the temperature (TTDC). ii. The temperature of the mixture may be considered to be that at TDC (TTDC) but raised by a DTp due to the combustion energy release by the pilot distributed uniformly over the entire mixture, i.e. (TTDC + DTp). iii. The temperature of the mixture at the commencement of ¯ame propagation is probably less than that represented in (ii) but higher than (i), since neither the energy release by the pilot then needs to be complete, nor is its distribution throughout the charge instantaneous. A more realistic value of the gaseous fuel±air mixture temperature can be represented as (TTDC + aDTp). The value of a, which is less than unity, will be aected by the highly nonuniform temperature distribution, heat losses and the fact that the pilot fuel may not be fully oxidized at the time of commencement of ¯ame propagation within the unburned gas fuel±air mixture. Fig. 5 shows that a value of 0.40 for a does produce approximately the same calculated ¯ame temperature for the observed experimental limit data. Accordingly, a simple procedure can be suggested to estimate the ¯ame spread limit in an engine for a range of test conditions when at least a single experimental value for the limit is available for a speci®ed set of conditions. For such a known value (with known intake ¯ow rates, composition, charge temperature, pilot quantity, engine speed and engine compression ratio) the mean temperature of the charge at top dead centre (TTDC) is calculated together with the contribution of the full combustion energy release of the pilot fuel, i.e. (DTp) using conventional classical thermodynamic methods. The corresponding ¯ame temperature of the gaseous fuel±air mixture at the known limit is then calculated thermodynamically on the basis of the initial mixture temperature to be equal to (TTDC + 0.4DTp). Assuming that limit mixtures for dierent operating conditions have the same value of this ®nal combustion temperature [9], the corresponding values of the ¯ame spread limit under the alternative conditions can then be calculated. Fig. 6 shows that the ¯ame spread limits (FSL) can be predicted quite well on this
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Fig. 5. Variations of the calculated adiabatic ¯ame temperatures at the ¯ame spread limit with pilot quantity when calculated on the three bases describes.
basis for methane operation at varying pilot quantities. The limit value for a pilot quantity of 0.40 kg/h is assumed to have been known. Limit values calculated on the basis of temperatures calculated in accordance with methods (i) and (ii) stated earlier, produced, as expected, much poorer agreement with the corresponding experimental values. For our experimental data, the error involved in estimating these limits was of a similar order to that associated with determining the limits experimentally (26%). The conditions under which the charge of the engine exists during ¯ame initiation and spread are highly turbulent and quite dierent from those of a quiescent similar mixture contained in the test apparatus commonly used for the determination of the ¯ammability limit values [10]. Nevertheless, there are common features of the transport and chemical phenomena
Fig. 6. Variations of measured and predicted ¯ame spread limits with pilot quantity.
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Fig. 7. Variations of the observed ¯ame spread limits and the corresponding quiescent lower ¯ammability limits with pilot quantity.
in these two types of limits to suggest that they may be related. Accordingly, the experimentally established ¯ame spread limit in the engine was compared with the corresponding ¯ammability limit for the same initial temperature and pressure at the time of commencement of ¯ame propagation. As shown in Fig. 7, the observed ¯ame spread limits (FSL), although following similar trends to the corresponding quiescent ¯ammability limits (LFL) determined for engine-like conditions, are signi®cantly higher. A ratio of around 2.0 was observed under dierent intake temperatures, pilot quantities and pilot injection timings, as shown in Fig. 8. It was also shown experimentally [11] that high levels of turbulence and velocity of a ¯owing homogeneous mixture of methane and air in a pipe can elevate the ¯ammability limit values of methane by the same order. This would suggest that in the absence of directly measured values, the ¯ame
Fig. 8. Variations of the derived Limit Ratio (LFL./FSL) with Pilot Quantity for a methane fueled engine (A) operating at air intake temperature of 300K.
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spread limits (FSL) in a dual fuel engine may be estimated on the basis of approximately twice the quiescent ¯ammability limits (LFL) evaluated at the cylinder temperature at the time of commencement of ¯ame propagation, as described earlier (i.e. TTDC + 0.40 DTp). 6. Conclusions A ¯ame spread limit (FSL) in a dual fuel engine has been identi®ed and its value established experimentally. The values of the ¯ame spread limit in an engine under dierent operating conditions can be predicted on the basis that they correspond to mixtures of the same ®nal combustion temperature for a calculated initial mixture temperature of (TTDC + 0.4 DTp). A single known condition for the engine can establish the value of this ®nal temperature. The measured ¯ame spread limits in an engine have been observed to correspond to around twice the quiescent lower ¯ammability limit calculated at a mixture temperature of (TTDC + 0.4 DTp) for the conditions examined. Acknowledgements The ®nancial assistance of Natural Sciences and Engineering Research Council of Canada is acknowledged. The contributions of Drs Z. Liu and D. Azzouz and Mr K. Burn to this investigation are also acknowledged. References [1] S.H. Turner, C.S. Weaver, Dual fuel natural gas/diesel engines: technology, performance and emissions. Gas Research Institute Technical Report no. 94/0094, 1994. [2] M.A. Elliot, R.E. Davis, Dual fuel combustion in diesel engines, Ind. Engng Chem. 43 (1951) 2854±2863. [3] G.A. Karim, The dual fuel engine. in: R.L. Evans (Eds.), Automotive Engine Alternatives, Plenum Press, New York, 1987. [4] X. Ding, P.G. Hill, Emissions and fuel economy of a prechamber diesel engine with natural gas dual fueling. SAE Paper no. 860069, 1986. [5] G.A. Karim, An examination of some measures for improving the performance of gas fuelled diesel engines at light load. SAE Paper no. 912366, Trans. SAE, 1991. [6] Z. Liu, G.A. Karim, Simulation of combustion processes in gas-fuelled diesel engines, J. Power Energy, Proc. Inst. Mech. Engrs 211 (A2) (1997) 159±169. [7] G.A. Karim, I. Wierzba, Experimental and analytical studies of the lean operational limits in methane fuelled spark ignition and compression ignition engines. SAE Paper No. 891637, 1989. [8] O.A. Badr, N. Elsayed, G.A. Karim, An investigation of the lean operational limits of gas fueled spark ignition engines, J. Energy Res. Technol., ASME Trans. 118 (1996) 159±163. [9] I. Wierzba, Shrestha S.O. Bade, G.A. Karim, An approach for predicting the ¯ammability limits of fuel/diluent mixtures in air, J. Inst. Energy 69 (1996) 122±130. [10] H.F. Coward, G.W. Jones, Limits of the ¯ammability of gases and vapors. Bulletin no. 503, US Bureau of Mines, 1952. [11] G.A. Karim, I. Wierzba, B. Soriano, The limits of ¯ame propagation within homogeneous streams of fuel and air, J. Energy Res. Technol. ASME Trans. 108 (1986) 183±187.
Applied Thermal Engineering 19 (1999) 1071±1080
An examination of the ¯ame spread limits in a dual fuel engine O. Badr 1, G.A. Karim *, B. Liu The University of Calgary, Department of Mechanical Engineering, Calgary, Alta., Canada, T2N 1N4 Received 3 March 1998; accepted 12 October 1998
Abstract The performance of a gas-fuelled diesel engine (dual fuel) is examined at light load and an eective threshold limit to the combustion of the gaseous fuel through bulk ¯ame spread is identi®ed. The relationship of such a limit to some of the key operating parameters is then discussed. A comparison between the measured values of the limit with those corresponding to the lower ¯ammability limits of the gaseous fuel when evaluated under the prevailing cylinder conditions during pilot diesel fuel ignition showed similar trends. It is suggested that such a similarity may form a basis for estimating the lean operational limits for dual fuel combustion in engines. A simple approach for estimating the limiting equivalence ratio for the apparent bulk ¯ame spread limit is described for a methane-fuelled dual fuel engine. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The use of alternative gaseous fuels in engines for the production of power has been increasing worldwide. This has been prompted by the cleaner nature of their combustion compared with conventional liquid fuels as well as their relative increased availability at attractive prices. Their use in engines is expected to increase with the ever tightening of exhaust emission regulations. Diesel engines, with appropriate relatively simple conversion, can be made to operate on gaseous fuels eciently. Such engines, which are called `dual fuel engines', usually have the gaseous fuel mixed with the air in the engine cylinders, either through direct mixing in the intake manifold with air or through injection directly into the cylinder. The resulting mixture * Corresponding author. Tel.: +1-403-220-5775; fax: +1-403-282-8406; e-mail: [email protected] 1 Currently at Qatar University, Doha, Qatar. 1359-4311/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 8 ) 0 0 1 0 8 - 2
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after compression is then ignited through the injection of a small amount of diesel fuel (the pilot) in the usual way. This pilot liquid fuel can autoignite readily to provide ignition sources for subsequent ¯ame propagation within the surrounding gaseous fuel mixture. Most current dual fuel engines are made to operate interchangeably, either on gaseous fuels with diesel pilot ignition or wholly on liquid fuel injection as a diesel engine. Accordingly, a dual fuel engine tends to retain most of the positive features of diesel operation. It may even surpass occasionally those of the diesels, producing higher power outputs and eciencies. This is achieved without signi®cant smoke or particulates emission and with reduced NOx production [1], while having reduced peak cylinder pressures and quieter operation. On the other hand, dual fuel operation may produce knocking, particularly with very high power outputs or raised intake temperatures, even with the knock-resistant fuel, methane. Fortunately, the knocking region with natural gas, which is mostly made up of methane, is out of most common operations, unless highly turbocharged engines are used, or the natural gas composition includes signi®cant concentrations of higher hydrocarbon fuel components. A notable feature of dual fuel engine operation is its inferior performance at very light load, especially when using small pilots, manifested by increased speci®c energy consumption, cyclic variations and higher emissions of CO and HC relative to the corresponding diesel operation. These limitations arise primarily from the fact that ¯ames originating from ignition regions within the pilot envelope cannot propagate fast and far enough within the time available to consume the entire fuel-lean mixture [2±4]. Such performance can be improved through measures such as the use of high enough gaseous fuel concentrations to permit ¯ame propagation, larger pilots, heating the intake charge, partial throttling the air component, variable pilot injection timing, optimal strati®cation of gaseous fuel admission and lower engine speeds [5]. Successful implementation of such measures requires a knowledge of the limiting concentration of the gaseous fuel in air beyond which favourable operation begins to be encountered. The present contribution examines the performance of a dual fuel engine at light load and identi®es an eective threshold limit to the consumption of the gaseous fuel with bulk ¯ame spread. The relationship of such a limit to some of the key operating parameters is then discussed. A comparison is also made between the measured values and those corresponding to the lower ¯ammability limits of the fuel evaluated at the cylinder temperature and pressure at the time of pilot ignition. It is shown that the similar trends between these two sets of limits suggest that they may form a basis for estimating the lean limits for dual fuel combustion and satisfactory engine operation.
2. Apparatus The experimental results in this investigation were obtained from two single cylinder, four stroke, watercooled, direct injection, normally aspirated laboratory dual fuel engines having 105 mm bore and 152.5 mm stroke with either a compression ratio of 14.2 and pilot injection timing of 208 for engine A or 14.7 and 188, respectively, for engine B. The intake mixture temperature was varied either through cooling for engine A from 293 to 250 K or through
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heating for engine B from 288 to 400 K. The fuel employed was technically pure methane and the diesel fuel had a cetane number of around 45. The engines were run at 1000 rpm. The total equivalence ratio during dual fuel operation which accounted for the contributions of the methane and diesel pilot fuels was calculated as the measured mass air ¯ow rate relative to the corresponding mass rate required for the stoichiometric combustion of the two fuel components. 3. Lean combustion at light load The autoignition of the injected pilot liquid diesel fuel provides ignition centres for turbulent ¯ame propagation throughout the lean homogeneous gaseous fuel±air mixture. Engine performance at light load tends to improve with increased admission of the gaseous fuel. The extent of the improvement appears to be dependent on the total equivalence ratio (i.e. based on the pilot and gaseous fuels). Fig. 1 shows the variations with total equivalence ratio of the concentrations of unconsumed methane and carbon monoxide in the exhaust gas for the engine for dierent pilot quantities. It can be seen that there is a limiting equivalence ratio beyond which the exhaust emissions of the carbon monoxide and the unconverted methane become virtually unaected by the pilot quantity. This is indicative of the equivalence ratio limit for successful ¯ame propagation from the pilot ignition centres. Fig. 2 shows schematically the typical variations in the extent of exhaust emissions of carbon monoxide and methane with the overall equivalence ratio for a ®xed pilot quantity. Broad operational regions may be identi®ed. The ®rst region is associated with extremely low gaseous fuel admission where the exhaust emissions of carbon monoxide and the fraction of the methane consumed are very small. In the second region, following an increased admission of the gaseous fuel, the consumption of the methane and the production of carbon monoxide begin to increase rapidly with the continued increased admission of the methane. Later on,
Fig. 1. The variations of the exhaust gas concentrations of methane and carbon monoxide with total equivalence ratio for dierent pilot fuel quantities at ambient intake conditions and 1000 rpm.
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Fig. 2. Schematic variations of the uncoverted methane and carbon monoxide concentrations in the exhaust with total equivalence ratios showing the dierent operational regions.
these begin to decrease in regions III and IV. These limiting values of equivalence ratio can be identi®ed as f1, f2 and f3 which may signify the start of signi®cant local partial oxidation, ¯ame initiation and the spread of propagating ¯ames within the gaseous fuel±air charge, respectively. The complex chemical and physical interactions that take place to produce these regions require the consideration of a number of related processes. These would include the preignition reaction activity of the gaseous fuel±air mixture during compression, the pilot injection processes and subsequent formation of the ¯ammable envelope, progressive reactions during the ignition delay of the pilot, formation of ignition centres and subsequent reactions with the gas±air mixture that may lead to partial or complete ¯ame propagation [6]. When operating with very fuel lean mixtures at light load most of the energy release comes from the combustion of the pilot and the gaseous fuel entrained within its envelope as well as adjacent reacting zones where high temperature may evolve. The contribution of the bulk surrounding lean gaseous fuel±air mixture to the energy release remains small [5]. For less lean mixtures, the concentration of the gaseous fuel may become suciently high to permit ¯ame propagation throughout the entire charge within the time available to contribute signi®cantly at a more gradual rate to the overall energy release [3]. Generally, the oxidation of a fuel such as methane proceeds sequentially via the formation of formaldehyde followed by carbon monoxide and the subsequent conversion to carbon dioxide and water vapour. For suciently fuel rich mixtures yielding high temperatures, good conversion of the methane±air mixture to completion takes place with little carbon monoxide and unconverted methane appearing in the exhaust. For less rich mixtures producing on combustion moderately high temperatures, a substantial amount of the carbon monoxide produced cannot be converted in the time available to carbon dioxide. However, for suciently lean mixtures, the charge temperature may be so low that no signi®cant reactions proceed, leaving the bulk of the methane unconverted and producing insigni®cant amounts of carbon monoxide in the exhaust. In region I of Fig. 2, associated with very low equivalence ratios, carbon monoxide is produced at very low levels and comes mainly from the combustion of the pilot. The contribution made by the surrounding zone of the gaseous fuel±air mixture is very small. In
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region II, some of the exhaust carbon monoxide originates increasingly from the preignition reactions of the gaseous fuel within the unburned zone, yet they are incapable of leading to ¯ame propagation within the time available, in spite of the presence of ignition centres. Further increases in equivalence ratio beyond the value f2 permit some ¯ame propagation within the methane±air mixture. In region III, further increases in the admission of the gaseous fuel produces extentions to the size of both the pilot envelope and the adjacent reacting zone and extends the ¯ame propagation into a larger fraction of the cylinder charge. With increased gaseous fuel admission the ¯ame propagation continues to extend into further regions of the charge until at f3 it extends essentially to all parts of the combustion chamber. Further increases in the gaseous fuel concentration within region IV produce essentially proportionally high rates of heat release, leading to high cylinder pressures and increased power output [6].
4. Combustion limits The existence of combustion limits in gas fuelled diesel engines has been recognized by dierent workers over the years. Elliot and Davis [2] reported such apparent limits in a dual fuel engine when using a number of dierent gaseous fuels. Karim and Wierzba [7] showed that the operational limits in a gas-fuelled spark ignition engine were amenable to correlation in terms of the charge mean temperature at the time of passing the spark. Badr et al. [8] de®ned for a gas-fuelled spark ignition engine two operational mixture limits and related them to the corresponding values under quiescent conditions calculated for the prevailing conditions within the cylinder at the time of passing the spark. A knowledge of the lean limits of combustion in gas-fuelled engines would obviously help in improving the design and operation of engines, particularly when embarking on the conversion of conventional diesel engines to gas fuelled operation. There is a need to establish whether simple guidelines for estimating such limits can be derived and to estimate their values in terms of operating conditions for a speci®c engine. The volumetric concentration of the gaseous fuel in air at the ¯ame spread limit (FSL) which corresponds to the equivalence ratio f3 of Fig. 2, as indicated earlier, identi®es the boundary for the commencement of satisfactory engine operation and improved emissions. This limit represents the minimum concentration of the gaseous fuel in air for which ¯ame propagation appears to spread throughout the entire cylinder charge. Such a limit for a speci®c engine is obtained experimentally when the emissions of the unconverted gaseous fuel becomes essentially independent of the pilot quantity employed. On the other hand, the equivalence ratio of the charge associated with the observed peak value of the concentrations of carbon monoxide exhaust emissions, corresponding to f2 in Fig. 2, may be considered as indicative of the commencement of some limited ¯ame propagation into the adjacing mixture. These limits need to be calculated for the gaseous fuel on the basis of the air remaining after accounting for its reduction following the complete oxidation of the pilot fuel. This lean limit for ¯ame spread in the dual fuel engine, despite obvious dierences, is reminiscent of the limit for ¯ame propagation within corresponding quiescent mixtures commonly known as the lean ¯ammability limit (LFL). Accordingly, it can be suggested that
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Fig. 3. Variations of the experimentally established ¯ame spread limit (FSL) with the pilot quantity employed for methane operation at a compression ratio of 14.2:1 and 1000 rpm. The corresponding ¯ame initiation limit values are also shown.
the observed limit (FSL) in the engine should be relatable to the corresponding lower ¯ammability limit (LFL) of the gaseous fuel under temperature and pressure values similar to those of the gaseous fuel±air cylinder charge during pilot fuel ignition, albeit under quiescent conditions. Fig. 3 shows the observed variations of the ¯ame spread limits (FSL) with changes in the pilot quantity derived from experimental data for the test engine with an estimated uncertainty of 5%. The lowering of the limit with the increase in the pilot quantity is expected due to a number of contributing factors. These include a greater energy release on ignition, correspondingly improved pilot injection characteristics, a larger size of pilot mixture envelope with a greater entrainment of the gaseous fuel, a larger number of ignition centres requiring shorter ¯ame travels, higher rates of heat transfer to the unburned gaseous fuel±air mixture and an increased contribution of hot residual gases. The ¯ame initiation limit shown in Fig. 3, exhibits a similar trend. Fig. 4 shows similarly how the operational limit (FSL) of the two dual fuel engines employed for a ®xed pilot quantity varies with changes in the intake charge temperature. As expected, the limit is lowered with the increase in intake temperature. This
Fig. 4. Variations of the experimentally established ¯ame spread limit (FSL) with the intake temperature for the two engines employed.
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trend arises mostly from the corresponding increase in the mean charge temperature during pilot ignition. 5. Correlations of the observed limits It has been shown that homogeneous quiescent mixtures of most common gaseous fuels in air at their corresponding lean ¯ammability limit concentrations produce on combustion approximately the same ¯ame temperature for a wide range of temperatures and pressures [9]. The observed ¯ame spread limits in an engine may be tested to establish whether they can be correlatable on a similar basis. The gaseous fuel±air engine mixture is at a condition that corresponds to the average cylinder temperature and pressure at the time of ¯ame initiation (i.e. at the point of pilot ignition). These conditions which are a function of most engine operating variables such as intake temperature and pressure, compression ratio, pilot injection quantity and timing, heat transfer and the physical properties of the mixture were evaluated according to the following three dierent possible simplifying interpretations of the processes: i. The temperature is considered to be that following compression calculated at TDC conditions with the assumption that pilot ignition contributes negligibly little to the value of the temperature (TTDC). ii. The temperature of the mixture may be considered to be that at TDC (TTDC) but raised by a DTp due to the combustion energy release by the pilot distributed uniformly over the entire mixture, i.e. (TTDC + DTp). iii. The temperature of the mixture at the commencement of ¯ame propagation is probably less than that represented in (ii) but higher than (i), since neither the energy release by the pilot then needs to be complete, nor is its distribution throughout the charge instantaneous. A more realistic value of the gaseous fuel±air mixture temperature can be represented as (TTDC + aDTp). The value of a, which is less than unity, will be aected by the highly nonuniform temperature distribution, heat losses and the fact that the pilot fuel may not be fully oxidized at the time of commencement of ¯ame propagation within the unburned gas fuel±air mixture. Fig. 5 shows that a value of 0.40 for a does produce approximately the same calculated ¯ame temperature for the observed experimental limit data. Accordingly, a simple procedure can be suggested to estimate the ¯ame spread limit in an engine for a range of test conditions when at least a single experimental value for the limit is available for a speci®ed set of conditions. For such a known value (with known intake ¯ow rates, composition, charge temperature, pilot quantity, engine speed and engine compression ratio) the mean temperature of the charge at top dead centre (TTDC) is calculated together with the contribution of the full combustion energy release of the pilot fuel, i.e. (DTp) using conventional classical thermodynamic methods. The corresponding ¯ame temperature of the gaseous fuel±air mixture at the known limit is then calculated thermodynamically on the basis of the initial mixture temperature to be equal to (TTDC + 0.4DTp). Assuming that limit mixtures for dierent operating conditions have the same value of this ®nal combustion temperature [9], the corresponding values of the ¯ame spread limit under the alternative conditions can then be calculated. Fig. 6 shows that the ¯ame spread limits (FSL) can be predicted quite well on this
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Fig. 5. Variations of the calculated adiabatic ¯ame temperatures at the ¯ame spread limit with pilot quantity when calculated on the three bases describes.
basis for methane operation at varying pilot quantities. The limit value for a pilot quantity of 0.40 kg/h is assumed to have been known. Limit values calculated on the basis of temperatures calculated in accordance with methods (i) and (ii) stated earlier, produced, as expected, much poorer agreement with the corresponding experimental values. For our experimental data, the error involved in estimating these limits was of a similar order to that associated with determining the limits experimentally (26%). The conditions under which the charge of the engine exists during ¯ame initiation and spread are highly turbulent and quite dierent from those of a quiescent similar mixture contained in the test apparatus commonly used for the determination of the ¯ammability limit values [10]. Nevertheless, there are common features of the transport and chemical phenomena
Fig. 6. Variations of measured and predicted ¯ame spread limits with pilot quantity.
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Fig. 7. Variations of the observed ¯ame spread limits and the corresponding quiescent lower ¯ammability limits with pilot quantity.
in these two types of limits to suggest that they may be related. Accordingly, the experimentally established ¯ame spread limit in the engine was compared with the corresponding ¯ammability limit for the same initial temperature and pressure at the time of commencement of ¯ame propagation. As shown in Fig. 7, the observed ¯ame spread limits (FSL), although following similar trends to the corresponding quiescent ¯ammability limits (LFL) determined for engine-like conditions, are signi®cantly higher. A ratio of around 2.0 was observed under dierent intake temperatures, pilot quantities and pilot injection timings, as shown in Fig. 8. It was also shown experimentally [11] that high levels of turbulence and velocity of a ¯owing homogeneous mixture of methane and air in a pipe can elevate the ¯ammability limit values of methane by the same order. This would suggest that in the absence of directly measured values, the ¯ame
Fig. 8. Variations of the derived Limit Ratio (LFL./FSL) with Pilot Quantity for a methane fueled engine (A) operating at air intake temperature of 300K.
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spread limits (FSL) in a dual fuel engine may be estimated on the basis of approximately twice the quiescent ¯ammability limits (LFL) evaluated at the cylinder temperature at the time of commencement of ¯ame propagation, as described earlier (i.e. TTDC + 0.40 DTp). 6. Conclusions A ¯ame spread limit (FSL) in a dual fuel engine has been identi®ed and its value established experimentally. The values of the ¯ame spread limit in an engine under dierent operating conditions can be predicted on the basis that they correspond to mixtures of the same ®nal combustion temperature for a calculated initial mixture temperature of (TTDC + 0.4 DTp). A single known condition for the engine can establish the value of this ®nal temperature. The measured ¯ame spread limits in an engine have been observed to correspond to around twice the quiescent lower ¯ammability limit calculated at a mixture temperature of (TTDC + 0.4 DTp) for the conditions examined. Acknowledgements The ®nancial assistance of Natural Sciences and Engineering Research Council of Canada is acknowledged. The contributions of Drs Z. Liu and D. Azzouz and Mr K. Burn to this investigation are also acknowledged. References [1] S.H. Turner, C.S. Weaver, Dual fuel natural gas/diesel engines: technology, performance and emissions. Gas Research Institute Technical Report no. 94/0094, 1994. [2] M.A. Elliot, R.E. Davis, Dual fuel combustion in diesel engines, Ind. Engng Chem. 43 (1951) 2854±2863. [3] G.A. Karim, The dual fuel engine. in: R.L. Evans (Eds.), Automotive Engine Alternatives, Plenum Press, New York, 1987. [4] X. Ding, P.G. Hill, Emissions and fuel economy of a prechamber diesel engine with natural gas dual fueling. SAE Paper no. 860069, 1986. [5] G.A. Karim, An examination of some measures for improving the performance of gas fuelled diesel engines at light load. SAE Paper no. 912366, Trans. SAE, 1991. [6] Z. Liu, G.A. Karim, Simulation of combustion processes in gas-fuelled diesel engines, J. Power Energy, Proc. Inst. Mech. Engrs 211 (A2) (1997) 159±169. [7] G.A. Karim, I. Wierzba, Experimental and analytical studies of the lean operational limits in methane fuelled spark ignition and compression ignition engines. SAE Paper No. 891637, 1989. [8] O.A. Badr, N. Elsayed, G.A. Karim, An investigation of the lean operational limits of gas fueled spark ignition engines, J. Energy Res. Technol., ASME Trans. 118 (1996) 159±163. [9] I. Wierzba, Shrestha S.O. Bade, G.A. Karim, An approach for predicting the ¯ammability limits of fuel/diluent mixtures in air, J. Inst. Energy 69 (1996) 122±130. [10] H.F. Coward, G.W. Jones, Limits of the ¯ammability of gases and vapors. Bulletin no. 503, US Bureau of Mines, 1952. [11] G.A. Karim, I. Wierzba, B. Soriano, The limits of ¯ame propagation within homogeneous streams of fuel and air, J. Energy Res. Technol. ASME Trans. 108 (1986) 183±187.